1. Field of the Invention
The present invention is generally concerned with torsional damper devices, in particular those for automobile vehicle clutches, which comprise at least three coaxial elements: a driven hub, a hub plate driving the latter in rotation with lost motion, and a drive system comprising a friction facing support disk, these coaxial elements being mounted to rotate relative to each other against spring means having at least two degrees of stiffness and over at least two predetermined sectors of relative angular displacement limited by the angular travel of meshing means.
2. Description of the Prior Art
In practise, these meshing means are of two types known per se:
A first, so-called main meshing means in which angular displacement takes place against so-called main spring means of greater stiffness consists of at least one drive means adapted to move angularly between two abutment means.
In many cases the drive means comprise at least one axial spacer, more often than not a spacer between the friction facing support disk to which it is fastened and a rotary element constrained to rotate with the latter and held at a predetermined axial distance from it.
The two abutment means determining the circumferential angular displacement of each spacer conventionally comprise the substantially radial edges of a notch formed in rotary elements, the hub plate, for example, disposed axially between the friction facing support disk and the rotary element fastened to it.
A second, so-called secondary meshing means, in which the relative angular displacement takes place against so-called secondary spring means of lower stiffness, consists in two inter-engaging concentric toothed means featuring a predetermined angular clearance.
Thus when a torsional couple is applied from the driving part, the driving part during a first phase rotates relative to the driven part against the secondary spring means, of lower stiffness, until the limits of relative angular displacement of the secondary meshing means are reached.
If the torsional torque is such that these limits are reached, relative displacement of the driven part, meaning the hub, takes place beyond these limits in a second phase against the main spring means of higher stiffness which, given their stiffness relative to that of the secondary spring means, behave as a rigid block during the first phase.
If the limits of relative angular displacement of the main meshing means are reached, then total coupling is achieved between the driving and driven parts.
The relative angular displacement between the friction facing support disk and the hub therefore takes place in succession against two stages of spring means of different stiffness and friction means. In the known manner this makes it possible to filter vibrations, especially when the vehicle is in neutral. In practise the secondary spring means are placed on a circumference of smaller diameter than the main springs.
The advantage of a torsional damper service featuring two successive stiffness stages is to permit more gradual operation than a single stiffness stage, that is as provided by a single kind of spring means.
The spring means are conventionally coil springs disposed circumferentially in openings provided for this purpose in the various rotary elements used.
These springs have a linear displacement/force function. This implies that the function determined by the behaviour of a device of this kind, as expressed in terms of angular displacement/torque, features thresholds determining linear ranges of operation.
In practice, this results in failure to achieve progressive coupling of the various rotating parts.
Apart from the resulting inconveniences of use, the parts wear more quickly.
To remedy the problem of lack of progressive functioning of torsional damper devices with a single stiffness stage, there have been developed more complex devices with two stiffness stages and corresponding stages of relative displacement, the structure and the operation of which have been briefly described hereinabove, or even a greater number of stiffness stages.
It will be obvious that increasing the number of stiffness and relative displacement stages enhances progressive functioning of the device.
It does have a number of disadvantages, however:
On the one hand, the relative complexity of the structure and thus the cost are increased.
On the other hand, the overall axial dimension of the device tends to increase, to provide room for the various spring means.
Accommodating the spring means in openings cut into the rotary elements tends to weaken these elements. In practise, this results in mandatory limits as to the multiplication of the number of stiffness stages.
An object of the present invention is a torsional damper device achieving the required progressive operation without entailing the above-mentioned disadvantages.